1. Field of the Invention
The present invention relates to production techniques for the manufacture of silicon single crystals by the Czochralski method (hereinafter referred to as the CZ method).
2. Background Art
Oxygen in a silicon wafer improves the operating characteristics of semiconductor devices by trapping contaminant atoms brought into the wafer during device fabrication processes (intrinsic gettering). Thus, the oxygen concentration in a silicon single crystal has to be kept within a given range throughout the constant diameter portion of the crystal.
Oxygen introduced into the silicon single crystal is derived from contact of the surface of the quartz crucible and the molten silicon. In the conventional CZ method, the oxygen concentration is not uniform in the pull direction. The reasons for this non-uniformity are believed to be as follows: (1) the change in contact area between the quartz crucible and the melt due to decreased volume of melt in the crucible as the crystal grows, (2) a change of the amount of oxygen eluted from crucible due to temperature changes resulting from the furnace environment during crystal pulling, and (3) a change in oxygen transport efficiency into the crystal due to changes in melt flow. However, no quantitative experimental support for these three factors have been reported, so their contribution to the oxygen concentration in the crystal is still not clear. On the other hand, in actual production of silicon single crystals, several methods have been proposed to alter the oxygen concentration with respect to solidification rate or crystal length. These methods include adjusting the crucible rotation rate, the pressure in the furnace, the flow rate of atmospheric gas, application of static magnetic fields, or combinations of several of these parameters, hereinafter referred to as “operational parameters.” Use of such parameters is disclosed in Japanese unexamined published patent applications H05-262593, H06-172081, H09-157085, and H10-167881.
It is theoretically possible to adjust the oxygen concentration in the crystal by adjusting one or more of the operational parameters. To accomplish this, however, crystals must be pulled under different conditions of operational parameters to establish a relationship between oxygen concentration and the various operational parameters. Thus, a great deal of effort is required to determine satisfactory adjustment procedures of the operational parameters during crystal growth. However, as shown in published application H09-157085, costly redetermination of vexatious operational parameters associated with component deterioration or product change are necessary, for example, in the case that a quality standard of the crystal is changed, a production furnace is replaced, a furnace part such as insulation or heater is changed, or the amount of molten feedstock is increased. Even without such changes, when the furnace ages, or the temperature environment within the furnace changes due to the deterioration of furnace members etc., the crystal oxygen concentration is affected thereby. Additionally, the recent scale up of silicon single crystal diameters from 8 to 12 inches which, of course, also involved scale up of the furnace, has exponentially increased both the cost as well as the time spent in experimental crystal pulling in order to satisfactorily adjust oxygen concentration. Thus, the conventional trial and error method leaves much to be desired when new quality standards or changes of operational environment are necessitated.